What is the easiest way to smooth the intersection of two planes?
Create a plane, enter edit mode (Tab) and select one of the edges.



Extrude (E) and pull up in the z axis.



Select the edge at the intersection and press Ctrl B to activate the Bevel tool. Moving the mouse will show the effect of the bevel.  Control the amount of cuts with the mouse wheel.
(or you can use the controls found on the toolshelf)